Some electronic systems are configured by connecting a plurality of electronic devices that can transmit/receive data by serial communications in series to an electronic device being a host controller. In such electronic systems, for example, a plurality of electronic devices being communication counterparts are connected to a serial communication signal line of one communication channel, and communications are performed while switching the electronic devices being the communication counterparts. In order to clarify the communication counterpart, a chip select signal line (CS signal line) may be provided separately from the serial communication signal line. However, in this case, the signal line increases in number, and the configuration of the electronic system becomes complicated.
Therefore, packet communications, which use a packet obtained by adding identification information for identifying the communication counterpart to data to specify the communication counterpart, are effective. In the packet communications, for example, in the electronic system in which two electronic devices are connected in series to the host controller, in order to transmit a packet to the electronic device located at a subsequent stage, the electronic device located at a previous stage relays the packet. Therefore, it is possible to build the electronic system that suppresses the number of signal lines and a signal wire length to a minimum.
U.S. Pat. No. 6,349,389 discloses a communication system configured by a slave relay station for receiving a serial communication signal which is connected to a host controller and another slave relay station further connected in series to the slave relay station. In this system, the host controller can communicate to/from each of the slave relay stations in a transparent manner.
U.S. Pat. No. 6,834,321 discloses a control system in which a plurality of electronic devices are connected in series to a host controller. When receiving a packet that is not addressed to itself, each of the electronic devices switches an input signal and relays the packet to another electronic device connected to the subsequent stage.
The electronic system in which the plurality of electronic devices are thus connected in series to the host controller can be configured with a small number of signal lines and a short signal line length.
In the electronic system in which the plurality of electronic devices are connected in series to the host controller, a frame including at least one packet is relayed to the electronic device being the communication counterpart in order from the electronic device close to the host controller to the electronic device being the communication counterpart. A frame transmitted from the electronic device being the communication counterpart to the host controller is also relayed by the electronic device located therebetween.
In order to establish communications between the respective electronic devices, the communications need to be performed at the same communication speed. Therefore, if the electronic device for relaying the frame changes the communication speed, the communications cannot be performed because the communication speed differs between the electronic devices. With the conventional configuration, the communication speed cannot be switched dynamically.